German patent specification DE 44 45 060 C1 discloses a circuit breaker comprising a bypass circuit (not elaborated upon there), by means of which the circuit breaker is automatically tripped if—for any reason whatsoever—the electronic tripping device does not trip the circuit breaker in spite of the fact that settable parameters have been exceeded, especially the tripping current and the associated delay time.
German patent application DE 199 27 030 A1 discloses a circuit breaker comprising an electronic, microprocessor-controlled tripping device and a bypass circuit (not elaborated upon there) which serve to effectuate an automatic tripping of the circuit breaker when the preset tripping current has been exceeded without tripping, and also comprising a watchdog circuit to monitor the function of the microprocessor. The bypass circuit is connected to the watchdog circuit and it has switching means (not elaborated upon there) that serve to control the current-dependent and time-dependent response characteristic curve of the bypass circuit as a function of the failure of a functional area of the microprocessor indicated by the watchdog circuit.
U.S. Pat. No. 5,214,560 discloses a watchdog monitor that functions as a bypass circuit for the microprocessor of the electronic tripping device of a circuit breaker. The monitoring circuit is made up of discrete components and consists of the serial connection of a high pass filter, of a transistor switch, of a timing circuit with a charging capacitor and of a voltage comparator. The properly operating microprocessor continuously emits watchdog pulses via the high pass filter to the transistor switch, whose output pulses continuously discharge the timing circuit, so that voltage cannot build up in the timing circuit since this could cause the comparator to toggle. If, in contrast, the watchdog pulses are absent due to a failure, then the charging capacitor soon reaches a voltage level that causes the comparator to toggle and, via its output, in turn, a tripping coil becomes excited, as a result of which the circuit breaker opens. A transistor switch arranged at the input of the high pass filter allows the watchdog pulses to be short-circuited so that an error function of the microprocessor can be simulated in order to test the bypass circuit.